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United States Patent |
6,262,700
|
Ueoka
|
July 17, 2001
|
Method for driving plasma display panel
Abstract
Focusing attention on a new fact that a difference in the operating margin
between sub-fields becomes remarkable when preliminary discharge is not
provided for all sub-fields, but is thinned out, the operating margin for
a plasma display panel will be improved by restricting the difference in
the operating margin.
In the case of the thinned preliminary discharge system, particularly the
dependence of maintenance blanking characteristics on the maintenance
pulse number becomes remarkable and as a result, the operating margin
difference among the sub-fields becomes remarkable, and therefore,
parameters for blanking pulse of the sub-field during the maintenance
blanking period are set in conformity with the maintenance pulse number
(number of times of emission) for each sub-field in order to restrain this
operating margin difference.
Inventors:
|
Ueoka; Mitsuo (Tokyo, JP)
|
Assignee:
|
NEC Corporation (Tokyo, JP)
|
Appl. No.:
|
252887 |
Filed:
|
February 18, 1999 |
Foreign Application Priority Data
| Feb 25, 1998[JP] | 10-042707 |
Current U.S. Class: |
345/68; 345/37; 345/63 |
Intern'l Class: |
G09G 003/28 |
Field of Search: |
345/37,41,42,60-72
315/169.4
|
References Cited
U.S. Patent Documents
5420602 | May., 1995 | Kanazawa | 345/67.
|
5436634 | Jul., 1995 | Kanazawa | 345/67.
|
5541618 | Jul., 1996 | Shinoda | 345/60.
|
5724054 | Mar., 1998 | Shinoda | 345/60.
|
5835072 | Nov., 1998 | Kanazawa | 345/60.
|
5874932 | Feb., 1999 | Nagaoka et al. | 345/60.
|
5889501 | Mar., 1999 | Sasaki et al. | 345/60.
|
5936355 | Aug., 1999 | Kim | 315/169.
|
6097357 | Aug., 2000 | Shinoda et al. | 345/63.
|
Foreign Patent Documents |
0 549 275 | Jun., 1993 | EP.
| |
0 657 861 | Jun., 1995 | EP.
| |
0 680 067 | Nov., 1995 | EP.
| |
Primary Examiner: Saras; Stevens
Assistant Examiner: Kumar; Srilakshmi
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas, PLLC
Claims
What is claimed is:
1. A method for driving a plasma display panel comprising the steps of:
dividing one field period displaying one screen of a plasma display panel
into a plurality of sub-fields, each sub-field comprising a write period,
a maintenance period and a maintenance blanking period;
setting a maintenance pulse number in each sub-field to a different value
to perform tonal display; and
setting a plurality of blanking parameters of blanking pulses during said
maintenance blanking period of each sub-field in conformity with said
maintenance pulse number of said sub-field.
2. A method for driving a plasma display panel according to claim 1,
wherein said plurality of blanking parameters comprises as least one of
said blanking pulse number, crest value, pulse width and rise time.
3. A method for driving a plasma display panel according to claim 1,
further comprising the step of providing preliminary discharge periods for
only a subset of said sub-fields.
4. A method for driving a plasma display panel according to claim 3,
wherein said sub-fields within one field are arranged in a sequence that
differs from a decreasing order of their maintenance pulse numbers.
5. A method for driving a plasma display panel according to claim 1,
wherein said blanking pulse comprises a bipolar pulse having positive and
negative polarities.
6. A method for driving a plasma display panel according to claim 1,
wherein said blanking pulse is supplied to a scanning electrode and a
common maintenance electrode.
7. A method for driving a plasma display panel according to claim 3,
wherein said preliminary discharge period has been provided for the
sub-field at the head.
8. A method for driving a plasma display panel according to claim 4,
wherein the sequence of said sub-fields within one field is configured
such that a sub-field arranged midway has a larger maintenance pulse
number than the other sub-fields.
9. A method for driving a plasma display panel according to claim 8,
wherein the sequence of said sub-fields is determined so that the
maintenance pulse number stepwise increases in said sub-fields from the
head to a middle and the maintenance pulse number stepwise decreases in
said sub-fields from the middle to an end.
10. A method for driving a plasma display panel according to claim 1,
wherein said blanking pulse is applied to a maintenance electrode and a
scanning electrode.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for driving a plasma display
panel, and more particularly, to a method for driving a color plasma
display panel capable of tonal display by dividing one field into a
plurality of sub-fields to set the number of times of emission for each
sub-field to different values.
2. Description of the Related Art
Conventionally, the tonal display on a plasma display panel has been
implemented by controlling a number of times of discharge (emission
luminance) during a maintenance period as shown in FIG. 11. More
specifically, one field (F), which displays one screen, is repeated 50 to
about 70 times a second, whereby screens of the respective fields are
stacked by means of afterimages of a human eye and a flicker-free natural
image can be obtained. This one-field period is divided into a plurality
of sub-fields (SF), and these sub-fields are combined by varying a
maintenance pulse number (a number of times of discharge) during the
maintenance period of each sub-field to thereby implement tonal display.
In, for example, display of 64 shades of gray, as shown in FIG. 11, one
field is constituted by six sub-fields: SF1 to SF6, and a preliminary
discharge period (preliminary lighting period+blanking period) is provided
at the head of each sub-field, and subsequent to this period, there are
provided a write period and a maintenance period respectively. The
weighting is effected by reducing the number of times of discharge during
these maintenance periods by about 1/2 for each successive sub-field, from
the sub-field at the head (in SF1, the number of times of discharge is
assumed to be 32n where n is a positive integer).
When the foregoing sub-field is selected within one frame for maintenance
discharge in accordance with this method, the emission luminance can be
controlled by the number of times of maintenance discharge in the
sub-field selected, and therefore, the display of 64 shades of gray can be
implemented.
In this respect, FIG. 12 is a sectional view showing a general plasma
display panel. In FIG. 12, reference numeral 1 designates a front
substrate; 2a, a scanning electrode; 2b, a maintenance electrode; 3, a bus
electrode; 4, a dielectric layer; 5, a rear substrate; 6, a data
electrode; 7, a white dielectric body; 8, fluorescent material; and 9, a
discharge cell respectively.
If the preliminary discharge periods are provided at the heads of all the
sub-fields as described above, preliminary discharge occurs at least six
times even in a non-display portion to cause light emission over the
entire screen. This emission causes black float particularly in a dark
place, thereby deteriorating the contrast. Also, if the sub-fields are
arranged simply in decreasing order of the weighting of emission luminance
(number of times of discharge) as shown in FIG. 11, a pseudo contour may
appear on displaying a moving image.
In order to suppress these defects, a driving sequence, as shown in FIG. 1,
is used (this driving sequence diagram in FIG. 1 is the same as that for
the present invention), in which this preliminary discharge is applied
once per field, and the sub-fields are not arranged simply in decreasing
order of the weighting of emission luminance (number of times of
discharge), but their sequence has been determined by contriving. In such
driving sequence, the preliminary discharge period is provided only for
the sub-field SF6 at the head, and the sub-field SF6 is constituted by the
preliminary discharge period, a write period, a maintenance period and a
maintenance blanking period. Each of the sub-fields SF1 to 5 other than
the sub-field SF6 is constituted by a write period, a maintenance period
and a maintenance blanking period.
In such driving sequence in which preliminary discharge is provided for all
sub-fields as shown in FIG. 11, the sequence, in which light is certainly
emitted over the entire screen at the beginning of each sub-field for
blanking, is adopted, and therefore, the presence or absence of wall
charge, which is caused by the presence or absence of maintenance
discharge of the sub-field in question, is bound to be erased, and does
not affect the next sub-field. In contrast, however, in such driving
sequence, in which the preliminary discharge is thinned out, as shown in
FIG. 1, the presence or absence of the maintenance discharge during a
maintenance period of the sub-field in question remains as a difference in
wall charge on the scanning electrode and maintenance electrode, and
therefore, the blanking characteristics of a maintenance blanking period
provided at the last of the sub-field becomes important as one of the
elements for determining the operating margin.
However, wall charge has conventionally been blanked by the use of
microdischarge using wall charge during the maintenance blanking period,
and therefore, the maintenance blanking period is susceptible to the
amount of wall charge, and the blanking characteristics easily becomes
unstable. Therefore, when it is adopted, such sub-field driving sequence
as shown in FIG. 1 is defective, in that the operating margin is lowered
and the yield is reduced as compared with the conventional method in which
all sub-fields are provided with preliminary discharge.
FIG. 13 shows the dependence of the operating margin in driving sequence in
sub-fields of FIG. 1 on the sub-field. The "minimum operating voltage" in
this figure is the minimum value of the drivable voltage, and the "maximum
operating voltage" is the maximum value of the drivable voltage. This
operable voltage range is the operating margin. When voltage exceeding
this operating margin is applied, an erroneous display occurs, and when
voltage below the operating margin is applied, a non-display portion
occurs. From this figure, it can be seen that the operating margin of the
sub-field next to a sub-field having low weighting of emission luminance
is lowered.
In other words, SF4, which is next to SF6 having the minimum emission
luminance, has the highest minimum operating voltage, and the lowest
maximum operating voltage. From this figure, therefore, it can also be
seen that the operating margin for the entire plasma display panel is
regulated by SF4 to be narrowed. The sub-field SF4, which is next to SF6
having the minimum emission luminance, has the minimum operating margin.
This is because the intensity of the maintenance discharge during a
maintenance period prior to the maintenance blanking period is affected by
the maintenance pulse number constituting the maintenance period.
As shown in FIG. 14, the maintenance discharge during the maintenance
period becomes stronger with the number of maintenance pulses PSUS to be
applied, and will be saturated. Therefore, when the number of maintenance
pulses is as small as 1 piece (case of n=1) like SF6, the maintenance
discharge does not become strong during the maintenance period. On the
other hand, at SF3, which follows SF1, the maintenance discharge becomes
strong because the number of maintenance pulses at SF1 is as sufficiently
great large as 32 pieces (case of n=1).
Since the number of the maintenance pulses differs depending on the
sub-field as described above, the intensity of the maintenance discharge
differs, and the amounts of wall charge which are produced by the
respective sub-fields during the maintenance period are different from one
another. Since these different wall charge have been blanked (neutralized)
during the maintenance blanking period having the same maintenance
blanking pulse, the blanking (neutralization) of the wall charge becomes
insufficient in a sub-field having a small number of maintenance pulses,
leading to decrease in the foregoing operating margin.
In this respect, a driving method in which the preliminary discharge is not
provided for all the sub-fields, but the number of times of preliminary
discharge per field is reduced in an attempt to enhance the display
contrast, is discussed in Japanese Patent Application Laid-Open Nos.
4-280289 and 7-49663. Also, a conventional example in which the waveform
of the blanking pulse has been contrived in order to obtain sufficient
blanking characteristics even if there are variations in the
characteristics of the discharge cell, is discussed in Japanese Patent
Application Laid-Open Nos. 8-30228 and 9-160522. They are aimed to
eliminate variations in the blanking characteristics within one field and
discharge cell.
SUMMARY OF THE INVENTION
The present invention is directed to on a new fact that in a case where the
preliminary discharge is not provided for all the sub-fields (case of
thinned preliminary discharge system in which preliminary discharge has
been thinned out), particularly the dependence of the maintenance blanking
characteristics on the maintenance pulse number becomes significant and as
a result, an operating margin difference among the sub-fields becomes
significant. One object of the present invention is to improve the
operating margin of the plasma display panel by restraining this operating
margin difference.
According to the present invention, there is provided a method for driving
a plasma display panel for dividing one field period displaying one screen
of a plasma display panel into a plurality of sub-fields, and setting a
number of times of light emission in each sub-field thus divided (setting
a maintenance pulse member in each sub-field) to different values for
tonal display, each of the foregoing sub-fields having at least a write
period, a maintenance period and a maintenance blanking period, wherein
parameters for blanking pulses (blanking parameters of blanking pulses)
during the foregoing maintenance blanking period are set in conformity
with the foregoing number of times of emission (maintenance pulse number)
during the foregoing maintenance period.
The foregoing maintenance blanking period is characterized in that a
plurality of blanking parameters constituting the maintenance blanking
period are at least one of the foregoing number of blanking pulses, crest
value, pulse width and rise time, and that the preliminary discharge
periods are thinned out and provided for a subset of sub-fields instead of
being provided for all sub-fields.
Further, the present invention is characterized in that the sequence of the
foregoing sub-fields within one field is arranged so as to be different
from the decreasing order of the number of times of emission, that the
foregoing blanking pulse is a bipolar pulse having positive and negative
polarities, and further that the foregoing blanking pulse is supplied to
the scanning electrode and a common maintenance electrode.
The operation of the present invention will be described. In the case of
the so-called thinned preliminary discharge system, in which preliminary
discharge is not provided for all sub-fields, particularly, the dependence
of the maintenance blanking characteristics on the maintenance pulse
number becomes significant and as a result, the operating margin
difference among the sub-fields becomes significant. Therefore, the
parameters for blanking pulses of the sub-fields during the maintenance
blanking period are set in conformity with the maintenance pulse number
(number of times of emission) for each sub-field in order to suppress the
operating margin difference.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing structure of one field for explaining an
embodiment according to the present invention;
FIG. 2 is a view showing examples of waveform at each portion for
explaining the operation in the embodiment according to the present
invention;
FIG. 3 is a partially enlarged view of FIG. 2;
FIG. 4 is a view showing a margin for driving voltage crest value for a
blanking pulse;
FIG. 5 is a view showing a margin for pulse width of a thick-width blanking
pulse;
FIG. 6 is a view showing a margin for pulse width of a fine-width blanking
pulse;
FIG. 7 is a view showing a margin for rise time of the thick-width blanking
pulse;
FIG. 8 is a view showing examples of waveform at each portion of another
embodiment according to the present invention;
FIG. 9 is a flow chart showing the operation of tonal display;
FIG. 10 is a flow chart showing the operation of maintenance blanking
discharge;
FIG. 11 is a view showing the structure of 1 field for explaining an
example of a conventional method for driving a plasma display panel;
FIG. 12 is a sectional view showing a general plasma display panel;
FIG. 13 is a view showing the dependency of the operating margin in the
sub-field structure of the thinned preliminary discharge system on the
sub-field; and
FIG. 14 is a view showing an aspect of maintenance discharge during the
maintenance period in the structure of FIG. 13 to the maintenance pulse.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Hereinafter, embodiments of the present invention will be described in
detail in conjunction with the drawings.
FIG. 1 shows the sub-frame structure of one field according to an
embodiment of the present invention and an example of driving sequence,
upon first glance this driving sequence causes this example to appear to
be the same as the foregoing conventional thinned preliminary discharge
system. However, parameters (pulse number, crest value, pulse width, rise
time, etc.) of blanking pulses during the maintenance blanking period in
each sub-field have been set so as to be different respectively in
response to the number of times of emission during the maintenance period
of each sub-field.
In order to perform tonal display, one field is structured so as to be
divided into six sub-fields: SF1 to SF6, and with the aim of improving the
contrast and preventing any pseudo contour from occurring during display
of a moving image, the number of times of preliminary discharge is set to
once a field, and the sequence of the sub-fields is changed from the
simple order of the weighting of emission luminance during the maintenance
period.
More specifically, as shown in FIG. 1, the sequence is: SF6 (weighting of
emission luminance: 1n), SF4 (weighting: 4n), SF2 (weighting 16n), SF1
(weighting: 32n), SF3 (weighting: 8n) and SF5 (weighting: 2n). The "n" is
a positive integer. However, this sequence is exemplary, and the driving
sequence is not limited thereto. In this respect, the weighting of this
emission luminance is effected on the basis of the number of maintenance
pulses constituting the maintenance period in the same manner as before.
The structure of each of these sub-fields will be described below. The
preliminary discharge period is provided only for the sub-field SF6 at the
head, and the sub-field SF6 comprises a write period, a maintenance period
and a maintenance blanking period which follow the preliminary discharge
period. Each of sub-fields SF1 to 5 other than the sub-field SF6 comprises
a write period, a maintenance period and a maintenance blanking period. In
this respect, the preliminary discharge period comprises a preliminary
lighting period and a preliminary blanking period which display the entire
display screen in the same manner as in the example of FIG. 11.
FIG. 2 shows a part of driving waveforms for sub-fields SF6 and SF4. As the
driving waveform, there are shown three types: a pulse train D which is
applied to the data electrode; pulse trains S0 and Sm which, of a
plurality of scanning electrodes, are applied to the 0-th one and m-th
one; and a pulse train C which is applied to the common maintenance
electrode.
In the present invention, the blanking pulse trains during this maintenance
blanking period are structured as below. FIG. 3 is a partially enlarged
view showing these blanking pulse trains. In FIG. 3, a first blanking
pulse P EC1 is applied to the maintenance electrode, subsequently a second
blanking pulse P ES2 is applied to the scanning electrode, a third
blanking pulse P EC3 and a fourth blanking pulse P ES4 are likewise
applied to each electrode respectively. Of these blanking pulses, the
first to third blanking pulses are called fine-width blanking pulses and
the fourth blanking pulse is called a thick-width blanking pulse.
The crest value, pulse width and rise time (negative polarity, negative
rise), which are parameters by which these blanking pulses are
characterized, are indicated by V1 to V4, .tau.1 to .tau.4 and t in FIG. 3
respectively. Since the optimum values for these values vary as a function
of sub-field, as shown in FIGS. 4 to 7, the parameters for blanking pulses
for each sub-field SF are determined in accordance with the tendency.
Next, the operation of the tonal display will be described with reference
to FIG. 9.
(1) The entire screen is caused to discharge and emit light once through a
preliminary lighting pulse PP during the preliminary discharge period,
positive charge, electrons, excitation atoms or molecules are generated
within discharge cells to activate the discharge cells, and wall charges
on the data electrode, scanning electrode and maintenance electrode are
neutralized (blanked) through preliminary blanking pulses P E1, P E2 and P
E3 to make preparations for causing the next write discharge with
stability (S1).
(2) Scanning pulses Pw are successively applied to a plurality of scanning
electrodes during the write period, and in synchronization therewith, a
data pulse PD corresponding to the displayed data is applied to generate
write discharge, for writing displayed data (S2).
(3) During the maintenance period, maintenance discharge is caused to occur
through a maintenance pulse P SUS in accordance with the data written for
displaying (S3).
(4) The maintenance discharge is stopped through P EC1, P ES2, P EC3 and P
ES4 during the maintenance blanking period, and wall charges on the data
electrode, scanning electrode and maintenance electrode are neutralized
(blanked) through maintenance blanking discharge to make preparations for
stabilizing write discharge for the next sub-field (S4).
(5) If this operation is not terminated (case of NO in S5), the procedure
will return to S2 again to repeat the steps to S5, and if the operation is
terminated (case of YES in S5), the process is terminated.
By the foregoing process, the tonal display can be performed by causing any
sub-field to emit light.
Next, the operation of maintenance blanking discharge (S4) will be
described with reference to FIG. 10.
For the neutralization (blanking) of wall charges during the foregoing
maintenance blanking period, optimum values as shown in FIGS. 4 to 7 for
the foregoing parameters differ for the respective sub-fields because the
sub-fields have different maintenance pulse numbers. Since parameters
(shown in FIG. 3) for blanking pulses constituting the maintenance
blanking period for each sub-field have been set to the optimum values
shown in FIG. 4, the wall charges could be neutralized (blanked) under the
optimum conditions in all the sub-fields (S11, S12). As a result, the
write characteristics of all the sub-fields were stabilized.
In this respect, as regards the maintenance blanking pulse number, it is
qualitatively known that when the pulse number is increased, the
maintenance blanking ability is improved. Since, however, the sub-fields
exhibit complicated behavior depending on their order of selection and
combination, the optimum pulse number was selected by cut-and-try methods.
The general view is that the blanking pulse number in the maintenance
blanking becomes large when the number of times of emission during the
maintenance period is small, and conversely that the blanking pulse number
becomes small when the number of times of emission is large.
FIG. 8 shows another embodiment according to the present invention, and in
this embodiment, the blanking pulse in the previous embodiment is
allocated to pulses of positive polarity and negative polarity and applied
to the scanning electrode and the maintenance electrode. Since the
amplitude of the blanking pulse can be reduced according to this driving
method, it becomes possible to lower the dielectric strength of the
driving circuit, and to reduce the circuit cost. According to this
embodiment, the blanking pulse is applied with plural and different crest
values, and therefore, the circuit becomes complicated. Therefore, this is
an important technique to provide low-priced products.
As described above, according to the present invention, when a blanking
period comprising a plurality of blanking pulses of the optimized
parameters is applied for each sub-field, the dependence of the operating
margin on the sub-field is eliminated, and the operating margin expands
even if the operation is caused to be performed only by one preliminary
discharge in one field.
Therefore, it is possible to manufacture a plasma display panel with
high-level display contrast in an excellent yield, and to reduce the cost.
Also, since the operating margin is large, it is possible to extend the
service life, and therefore, it is also possible to provide the products
with high reliability at low cost.
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